change selection type for left click-drag
On the off-chance anyone still wants to know the answer to this: Blender's selection tool now supports four different selection modes, which are accessed from the top icon in the new Tools menu, on the left side of the screen. The click-and-drag option is the new default setting, and is just called "Select"; the tool can also be set to box selection, lasso selection and circle selection modes. There is, iirc, a keyboard shortcut to cycle through the options — which is presumably what Wohe1 pressed accidentally — but I can't remember it off the top of my head. However, if you run up against the same problem he/she did, you can rectify it by restoring your preferred setting, which you can do by clicking and holding on the Select tool icon and waiting until a little menu pops up with the four options.